"This is a historical mission; it's not just a science mission," said Dr Edward Weiler, Associate Administrator for the Science Mission Directorate at Nasa.

"I maintain that it really attacks some very basic human questions that have been part of our genetic code since that first man or woman looked up into the sky and asked the question: 'are we alone?'."

Equipped with the largest camera ever launched into space, it is the first mission designed to find rocky worlds orbiting Sun-like stars. Planets located in a warm zone - known as the habitable zone - might host liquid water on their surfaces.

And where there is liquid water, scientists argue, there is at least the potential for life.

'Flea in headlights'

Of the 300-plus extrasolar planets discovered to date, only a handful are thought to be rocky like the Earth, and none are Earth-mass. The vast majority are gas giants like Jupiter or so-called ice giants like Neptune.

This is thought to be due to the technological limitations of current telescope technology, rather than a lack of Earth-mass planets in our galaxy.

THE KEPLER SPACE TELESCOPE

Will study more than 100,000 suns

Continuously for 4 to 6+ years

Tuned to see Earth-size planets

Will target the habitable zone

Also see Mars to Jupiter sizes

First results within 3 months

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Kepler will detect the periodic dimming of stars caused by transiting planets.

Some star systems are oriented in such a way that their planets cross in front of their stars, as seen from Earth's point of view. During these transits, the planets cause their star's light to slightly dim, or wink.

The orbiting observatory can register changes in brightness of only 20 parts per million.

"If Kepler were to look down at a small town on Earth at night from space, it would be able to detect the dimming of a porch light as somebody passed in front," said James Fanson, Kepler project manager at Nasa's Jet Propulsion Laboratory in California.

He commented: "Trying to detect Jupiter-size planets crossing in front of their stars is like trying to measure the effect of a mosquito flying by a car's headlight.

"Finding Earth-sized planets is like trying to detect a very tiny flea in that same headlight."

William Borucki, Kepler chief scientist from Nasa's Ames Research Center in California, said: "We will monitor a wide range of stars; from small cool ones, where planets must circle closely to stay warm, to stars bigger and hotter than the Sun, where planets must stay well clear to avoid being roasted."

Closer to home: Venus is pictured making a transit of our Sun

If the mission does find Earth-size planets in the habitable zones of stars, it should find them first around suns that are smaller than our own. In the case of small stars, this habitable zone is located closer in, so planets circling in this comfortable region will take less time to complete one lap.

This means it will take less time for Kepler to find them and for other ground telescopes to confirm their existence.

Any Earth-size planets orbiting in the habitable zones of stars the size of our Sun would take at least three years to be confirmed.

The telescope has a very large field of view, which it needs in order to observe the necessary large number of stars.

Kepler will stare at the same star field for the entire mission monitoring the brightnesses of more than 100,000 stars for the mission's three-and-a-half-year lifetime.

The spacecraft will be launched into an orbit where it will drift behind Earth as it circles the Sun. This will ensure our planet does not block the stars which need to be observed continuously by the observatory.

Getting answers

Dr Borucki suggested the mission could make major contributions to understanding other star systems, including the unresolved question of how they form. But he cautioned that the knowledge still to be accumulated was vast.

"If you look at the giant planets, we have found 340 of these. People say: 'Why would you need any more?'

"My guess is that by understanding giant planets and small planets we are going to make dramatic discoveries about other planetary systems. But I don't expect us to have all the answers in three-and-a-half years."

Finding Earth-size planets is one thing, confirming life - even simple microbial life - exists on them is another matter altogether; and will require future telescopes capable of seeing trace gasses in the planets' atmospheres that are a possible signature for biology.

"We certainly won't find ET but we might find ET's home by looking at all of these stars," said Bill Borucki.

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